Lightweight methods and compositions for well treating

ABSTRACT

Methods and compositions useful for hydraulic fracturing of subterranean formations that utilize relatively lightweight and/or substantially neutrally buoyant particles as particulate proppant material.

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 09/519,238, filed Mar. 6, 2000; which is acontinuation-in-part of U.S. patent application Ser. No. 09/085,416,filed May 27, 1998, which issued as U.S. Pat. No. 6,059,034; which is acontinuation-in-part of U.S. patent application Ser. No. 08/756,414,filed Nov. 27, 1996, now abandoned, and which also claims priority toDanish patent application S/N 1333/97 filed Nov. 21, 1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to subterranean formation treatmentsand, more specifically, to hydraulic fracturing treatments forsubterranean formations. In particular, this invention relates to use ofrelatively lightweight and/or substantially neutrally buoyant particlesas proppant material in hydraulic fracturing treatments.

2. Description of the Related Art

Hydraulic fracturing is a common stimulation technique used to enhanceproduction of fluids from subterranean formations. In a typicalhydraulic fracturing treatment, fracturing treatment fluid containing asolid proppant material is injected into the formation at a pressuresufficiently high enough to cause the formation or enlargement offractures in the reservoir. During a typical fracturing treatment,proppant material is deposited in a fracture, where it remains after thetreatment is completed. After deposition, the proppant material servesto hold the fracture open, thereby enhancing the ability of fluids tomigrate from the formation to the well bore through the fracture.Because fractured well productivity depends on the ability of a fractureto conduct fluids from a formation to a wellbore, fracture conductivityis an important parameter in determining the degree of success of ahydraulic fracturing treatment.

Hydraulic fracturing treatments commonly employ proppant materials thatare placed downhole with a gelled carrier fluid (e.g., aqueous-basedfluid such as gelled brine). Gelling agents for proppant carrier fluidsmay provide a source of proppant pack and/or formation damage, andsettling of proppant may interfere with proper placement downhole.Formulation of gelled carrier fluids usually requires equipment andmixing steps designed for this purpose.

SUMMARY OF THE INVENTION

In the disclosed method, the application of relatively lightweightand/or substantially neutrally buoyant particulate material as afracture proppant particulate advantageously may provide forsubstantially improved overall system performance in hydraulicfracturing applications. By “relatively lightweight” it is meant that aparticulate has a density that is substantially less than a conventionalproppant particulate material employed in hydraulic fracturingoperations, e.g., sand or having a density similar to these materials.By “substantially neutrally buoyant”, it is meant that a particulate hasa density sufficiently close to the density of a selected ungelled orweakly gelled carrier fluid (e.g., ungelled or weakly gelled completionbrine, other aqueous-based fluid, or other suitable fluid) to allowpumping and satisfactory placement of the proppant particulate using theselected ungelled or weakly gelled carrier fluid. For example, urethaneresin-coated ground walnut hulls having a specific gravity of from about1.25 to about 1.35 grams/cubic centimeter may be employed as asubstantially neutrally buoyant proppant particulate in completion brinehaving a density of about 1.2. It will be understood that these valuesare exemplary only. As used herein, a “weakly gelled” carrier fluid is acarrier fluid having minimum sufficient polymer, viscosifier or frictionreducer to achieve friction reduction when pumped down hole (e.g., whenpumped down tubing, work string, casing, coiled tubing, drill pipe,etc.), and/or may be characterized as having a polymer or viscosifierconcentration of from greater than about 0 pounds of polymer perthousand gallons of base fluid to about 10 pounds of polymer perthousand gallons of base fluid, and/or as having a viscosity of fromabout 1 to about 10 centipoises. An ungelled carrier fluid may becharacterized as containing about 0 pounds per thousand gallons ofpolymer per thousand gallons of base fluid.

Advantageously, in one embodiment use of substantially neutral buoyantparticulate material may eliminate the need for gellation of carrierfluid, thus eliminating a source of potential proppant pack and/orformation damage. Furthermore, a relatively lightweight particulatematerial may be easier to place within a targeted zone due to lessenedsettling constraints. Elimination of the need to formulate a complexsuspension gel may mean a reduction in tubing friction pressures,particularly in coiled tubing and in the amount of onlocation mixingequipment and/or mixing time requirements, as well as reduced costs.Furthermore, when treated to have sufficient strength (e.g., bysubstantially filling the permeable porosity of a porous particle withresin or hardener), the disclosed relatively lightweight proppantparticles may be employed to simplify hydraulic fracturing treatmentsperformed through coil tubing, by greatly reducing fluid suspensionproperty requirements. Downhole, a much reduced propensity to settle (ascompared to proppant particulates) may be achieved. In this regard, thedisclosed substantially neutral buoyancy proppant material may beadvantageously employed in any deviated well having an angle ofdeviation of between about 0 degree and about 90 degrees with respect tothe vertical. However, in one embodiment, the disclosed particulatematerial may be advantageously employed in horizontal wells, or indeviated wells having an angle with respect to the vertical of betweenabout 30 degrees and about 90 degrees, alternatively between about 75degrees and about 90 degrees. Thus, use of the disclosed relativelylightweight and/or substantially neutrally buoyant particulate materialsdisclosed herein may be employed to achieve surprising and unexpectedimprovements in fracturing methodology, including reduction in proppantpack and/or formation damage, and enhancement of well productivity.

In another embodiment, protective and/or hardening coatings, such asresins described elsewhere herein may be selected to modify or customizethe specific gravity of a selected base particulate proppant material,e.g., ground walnut hulls, etc. Modification of particulate specificgravity (i.e., to have a greater or lesser specific gravity) may beadvantageously employed, for example, to provide proppant particulatesof customized specific gravity for use as a substantially neutrallybuoyant particulate with a variety of different weight or specificgravity carrier fluids. In yet another embodiment, protective and/orhardening-type coatings may be optionally curable to facilitate proppantpack consolidation after placement. In this regard, curable resins areknow to those of skill in the art, and with benefit of this disclosuremay be selected to fit particular applications accordingly.

The disclosed relatively lightweight and/or substantially neutrallybuoyant particulate proppant materials may be employed with carrierfluids that are gelled, non-gelled, or that have a reduced or lightergelling requirement as compared to carrier fluids employed withconventional fracture treatment methods. In one embodiment employing oneor more of the disclosed substantially neutrally buoyant particulatematerials and a brine carrier fluid, mixing equipment need only includesuch equipment that is capable of (a) mixing the brine (dissolvingsoluble salts), and (b) homogeneously dispersing in the substantiallyneutrally buoyant particulate material.

In one embodiment, a substantially neutrally buoyant particulateproppant material may be advantageously pre-suspended and stored in astorage fluid (e.g. brine of near or substantially equal density), andthen pumped or placed downhole as is, or diluted on the fly.

In one respect, disclosed is a hydraulic fracturing method for a wellpenetrating a subterranean formation, including introducing a relativelylightweight and/or substantially neutral density particulate proppantmaterial into the well. Individual particles of the particulate materialoptionally may have a shape with a maximum length-based aspect ratio ofequal to or less than about 5. Individual particles may also beoptionally coated with protective materials such as resins and/orhardeners, for example, “2AC” phenol formaldehyde hardener from BORDENCHEMICAL. Examples of suitable relatively lightweight and/orsubstantially neutrally buoyant materials for use in aqueous basedcarrier fluids include, but are not limited to, ground or crushed nutshells, ground or crushed seed shells, ground or crushed fruit pits,processed wood, or a mixture thereof. Optional protective coatings forcoating at least a portion of individual particles of such relativelylightweight and/or substantially neutrally buoyant materials include,but are not limited to at least one of phenol formaldehyde resin,melamine formaldehyde resin, urethane resin, or a mixture thereof. Otheroptional coating compositions known in the art to be useful as hardenersfor such materials (e.g., coating materials that function or serve toincrease the elastic modulus of the material) may be also employed inconjunction or as an alternative to protective coatings, and may beplaced underneath or on top of one or more protective coatings. It willbe understood by those of skill in the art that such protective and/orhardening coatings may be used in any combination suitable for impartingdesired characteristics to a relatively lightweight and/or substantiallyneutrally buoyant particulate proppant material, including in two ormore multiple layers. In this regard successive layers of protectivecoatings, successive layers of hardening coatings, alternating layers ofhardening and protective coatings, etc. are possible. Mixtures ofprotective and hardening coating materials may also be possible.

In another respect, disclosed is a relatively lightweight and/orsubstantially neutrally buoyant fracture proppant material for use in ahydraulic fracturing treatment that is a ground or crushed walnut shellmaterial that is coated with a resin to substantially protect and waterproof the shell. Such a material may have a specific gravity of fromabout 1.25 to about 1.35, and a bulk density of about 0.67. In oneexemplary case, size of such a material may be about 12/20 US mesh size.In another exemplary case, sizes may range from about 4 mesh to about100 mesh. Advantageously, in some embodiments, such ground walnut shellsmay serve to attract fines and formation particles by their resinousnature. In one embodiment for the manufacture of such particles forproppant applications, an optional hardener may be applied to a groundwalnut shell material first followed by a urethane coating as describedelsewhere herein that may vary in amount as desired. For example, such acoating material may be present in an amount of from about 1% to about20%, alternatively from about 10% to about 20% by weight of total weightof individual particles. Alternatively, such a coating material may bepresent in an amount of from about 2% to about 12% by weight of totalweight of individual particles. Amount of resin may depend, for example,on price and application. In this regard, particulates may be firstsprayed or otherwise coated with a hardener, and a coating may beapplied to be about 12% by weight of total weight of the particle.

In one embodiment, the disclosed relatively lightweight particulateproppant material may be introduced or pumped into a well as neutrallybuoyant particles in, for example, a saturated sodium chloride solutioncarrier fluid or a carrier fluid that is any other completion orworkover brine known in the art, for example, having a specific gravityof from about 1 to about 1.5, alternatively from about 1.2 to about 1.5,further alternatively about 1.2, thus eliminating the need for damagingpolymer or fluid loss material. In one embodiment, such a material maybe employed as proppant material at temperatures up to about 150° F.,and pressures up to about 1500 psi. However, these ranges of temperatureand closure stress are exemplary only, it being understood that thedisclosed materials may be employed as proppant materials attemperatures greater than about 150° F. and/or at closure stressesgreater than about 1500 psi, it also being understood with benefit ofthis disclosure that core and/or layer material/s may be selected bythose of skill in the art to meet and withstand anticipated downholeconditions of a given application.

Advantageously, in one embodiment the low specific gravity of therelatively lightweight proppant material may be taken advantage of toresult in a larger width for the same loading (i.e., pound per squarefoot of proppant) to give much larger total volume and increased widthfor the same mass. Alternatively, this characteristic allows for smallervolumes of proppant material to be pumped while still achieving anequivalent width.

In yet another respect, disclosed is a method of fracturing asubterranean formation, including: injecting a particulate material intothe subterranean formation at a pressure above the fracturing pressureof the formation; wherein at least a portion of the individual particlesof the particulate material each include a first material selected fromat least one of ground or crushed nut shells, ground or crushed seedshells, ground or crushed fruit pits, processed wood, or a mixturethereof; and wherein at least a portion of the individual particles ofthe particulate material each includes a core component of the firstmaterial at least partially surrounded by at least one layer componentof second material, the second material including a protective orhardening coating.

In yet another respect, disclosed is a method of fracturing asubterranean formation, including: introducing a particulate materialsuspended in a carrier fluid into the subterranean formation at apressure above a fracturing pressure of the subterranean formation. Inthis method, at least a portion of the individual particles of theparticulate material may be substantially neutrally buoyant in thecarrier fluid and may include: a core component of a first materialselected from at least one of ground or crushed nut shells, ground orcrushed seed shells, ground or crushed fruit pits, processed wood, or amixture thereof; and at least one layer component of second materialsurrounding the core component, the second material including aprotective or hardening coating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representation of a particle of ground walnut hull materialaccording to one embodiment of the disclosed method.

FIG. 2 shows permeability versus closure stress for particulateaccording to one embodiment of the disclosed method.

FIG. 3 shows cell width versus closure stress for particulate accordingto one embodiment of the disclosed method.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

As used herein, the indefinite articles “a” and “an” connote “one ormore.”

Examples of types of materials suitable for use as relativelylightweight and/or substantially neutrally buoyant proppant particulatesinclude, but are not limited to, ground or crushed shells of nuts suchas walnut, pecan, almond, ivory nut, brazil nut, etc.; ground or crushedseed shells (including fruit pits) of seeds of fruits such as plum,peach, cherry, apricot, etc.; ground or crushed seed shells of otherplants such as maize (e.g. corn cobs or corn kernels), etc. processedwood materials such as those derived from woods such as oak, hickory,walnut, poplar, mahogany, etc. including such woods that have beenprocessed by grinding, chipping, or other form of particalization.Additional information on such materials and methods for use thereof maybe found in U.S. patent application Ser. No. 09/519,238 filed Mar. 6,2000 and entitled “Formation Treatment Method Using DeformableParticles,” which is incorporated herein by reference. Furtherinformation on materials and methods may also be found in the UnitedStates Patent Application entitled “Lightweight Methods and Compositionsfor Sand Control” by Harold D. Brannon, Allan R. Rickards, andChristopher J. Stephenson, filed on the same day as the presentapplication, and which is incorporated herein by reference. Furtherinformation on hydraulic fracturing methods and materials for usetherein may be found in U.S. Pat. No. 6,059,034, which is alsoincorporated herein by reference.

In one embodiment, specific gravity of such materials may range fromabout 0.4 to about 4, alternatively from about 0.8 to about 4. Inanother embodiment, specific gravity of such materials may range fromabout 0.4 to about 1.5, alternatively from about 0.5 to about 1.5. Inanother embodiment, specific gravity of such materials may range fromabout 0.5 to about 2, alternatively from about 0.5 to about 1.5,alternatively from about 1 to about 1.5, alternatively about 1.2. Itwill be understood that the foregoing embodiments are exemplary only andgreater or lesser values are also possible. With benefit of thisdisclosure, those of skill in the art will understand that selection ofsuitable specific gravity of such a proppant particulate will depend, inpart, on the specific gravity of the carrier fluid and on whether it isdesired that the selected proppant particle be relatively lightweight orsubstantially neutrally buoyant in the selected carrier fluid, and/orwhether or not it is desired that the carrier fluid be non-gelled ornon-viscosified.

It will be understood with benefit of this disclosure that suitablerelatively lightweight and/or substantially non-buoyant materials may bechipped, ground, crushed, or otherwise processed to produce particulatematerial having any particle size or particle shape suitable for use inthe methods disclosed herein. In one exemplary embodiment, particlesizes include, but are not limited to, sizes ranging from about 4 meshto about 100 mesh, alternatively from about 12 mesh to about 50 mesh. Inanother exemplary embodiment, particle sizes include, but are notlimited to, sizes ranging from about 8 mesh to about 40 mesh,alternatively from about 14 mesh to about 40 mesh. Shapes of suchparticles may vary, but in one embodiment may be utilized in shapeshaving maximum length-based aspect ratio values as described elsewhereherein for particles, and in one embodiment may have a maximumlength-based aspect ratio of less than or equal to about 5. Once again,the preceding ranges of values are exemplary only, and values outsidethese ranges are also possible.

Specific examples of suitable materials suitable for the relatively lowclosure stress embodiments described above include, but are not limitedto ground or crushed nut shells available from suppliers such as“COMPOSITION MATERIALS, INC.” of Milford, Conn.; “AGRASHELL, INC.” ofBath, Pa.; “BAROID”, and/or “CALIFORNIA NUT ASSOCIATION”. These productsinclude “walnut shell grit” available from “COMPOSITION MATERIALS,INC.”, “AD-3” ground walnut hulls from “AGRASHELL” (having a particlesize of about 12/20 mesh, a specific gravity of about 1.2, and a maximumlength-based aspect ratio of about 5), as well as “AD-6B” ground walnutshells (having a particle size of about 20/40 mesh, a specific gravityof about 1.2, and a maximum length-based aspect ratio of about 5). Suchground walnut hull material is available, for example, for use as ablasting media. FIG. 1 shows a simplified representation of a particle600 of ground walnut hull material having relative dimension ratio ofX:Y:Z. In one exemplary embodiment employing ground walnut hullmaterial, values of X, Y and Z may be expressed as a relative ratio(e.g., independent of any particular units of measurement employed) asfollows: X may be from about 1 to about 5; Y may be from about 1 toabout 5, and Z may be about 1. Alternatively, X may be from about 2 toabout 5; Y may be from about 2 to about 5, and Z may be about 1. Thesegiven ranges are exemplary only, and relative dimensional values of anyone or more of X, Y, and Z may fall outside these value ranges. Inalternate embodiments, ground nuts such as ground walnut hulls may beprocessed to have a substantially spherical or beaded shape as well.

In one exemplary embodiment, ground walnut hulls having a particle sizeof about 12/20 mesh and a maximum length-based aspect ratio of about 5may be employed as a proppant particulate. Such materials may be coatedfor use in these applications as described elsewhere herein.

In one embodiment, a multi-component relatively lightweight and/orsubstantially neutrally buoyant proppant particle may include a firstmaterial and at least one additional, or second, different material. Thefirst material and at least one second material may have differentvalues of in situ Young's modulus and/or be of differing composition.Alternatively, the first material and at least one second material mayhave similar or same values of in situ Young's modulus and/or be ofsimilar or same composition. In one embodiment, a second material may bepresent as a protective layer around a first material core, as describedfurther herein. In another embodiment, a second material may be presentto alter the overall modulus of a particulate formed therefrom, such asto function as a hardening material. For example, overall in situYoung's modulus of ground walnut hulls may be increased by coating suchparticles with a layer of relatively hard resin having a higher in situYoung's modulus. A single material may be present to perform bothprotective and hardening functions, or separate materials may be presentto perform each of these respective functions. As used herein, a “layer”refers to a second material that at least partially or completelysurrounds a first core material. A layer includes materials that adhereto or are otherwise disposed on the surface of a core material, and/orto those materials that are at least partially absorbed or permeatedinto a first core material.

In one embodiment, the two or more materials may be configured invirtually any maimer desired to form multi-component particles (forexample, as described elsewhere herein) to achieve varying overalldensity and/or hardness characteristics (or in situ Young's modulus) ofsuch particles, for example, to meet specific formation conditions.

In another embodiment, a first relatively lightweight and/orsubstantially neutrally buoyant core material may be coated or at leastpartially surrounded with at least one layer of a second material thatmay be selected to act to harden and/or isolate or protect the firstmaterial from adverse formation or wellbore conditions, for example soas to avoid exposure to acids or other workover/drilling fluids, toavoid saturation with liquids, provide longer fracture proppant packlife, etc. In this regard, any coating material known in the art andsuitable for imparting hardness and/or suitable for at least partiallyprotecting or isolating a first relatively lightweight and/orsubstantially buoyant core material as so described herein may beemployed. Examples of such hardening and/or protective materialsinclude, but are not limited to resins (e.g., urethane, phenolic,melamine formaldehyde, etc.) described for other use in otherembodiments elsewhere herein. With benefit of this disclosure, suitablecoating material/s may be selected by those of skill in the art toachieve or impart the desired qualities to a first relativelylightweight and/or substantially buoyant core material, consideringanticipated wellbore and/or formation conditions. Methods for coatingparticulates (e.g., fracture proppant particles, etc.) with materialssuch as resin are known in the art, and such materials are available,for example, from manufacturers listed herein. With regard to coating ofthe disclosed lightweight and/or substantially neutrally buoyantmaterials, coating operations may be performed using any suitablemethods known in the art. For example, low temperature curing methodsmay be employed if desired (e.g, using fast setting “cold set” or “coldcure” resins), where heating may be a problem, such as when coatingmaterials which may be sensitive to heat, like ground nuts or fruitpits. Alternatively, indirect heating processes may be employed withsuch materials when it is necessary to heat a coating material for cure.

Examples of resins that may be employed as layers for protective and/orhardening purposes include, but are not limited to, phenol formaldehyderesins, melamine formaldehyde resins, and urethane resins, low volatileurethane resins (e.g., these and other types of resins available fromBORDEN CHEMICAL INC., SANTROL, HEPWORTH of England), etc., and mixturesthereof. Specific examples of suitable resins include, but are notlimited to, resins from BORDEN CHEMICAL and identified as 500-series and700-series resins (e.g., 569C, 794C, etc.). Further specific examples ofresins include, but are not limited to, “SIGMASET” series lowtemperature curing urethane resins from BORDEN CHEMICAL (e.g.,“SIGMASET”, “SIGMASET LV”, “SIGMASET XL”), “ALPHASET” phenolic resinfrom BORDEN, “OPTI-PROP” phenolic resin from SANTROL, and “POLAR PROP”low temperature curing resin from SANTROL. Low temperature curing resinsmay be applied with little or no heat, which may be desirable whencoating heat-sensitive materials such as wood, nut shell material, ete.Alternatively, heat cured resins may be applied and cured using heatingmethods that are compatible with heat sensitive materials. For example,in one embodiment, ground walnut shells may be coated with SANTROL“OPTI-PROP” resin in a single coating step using indirect heat (e.g., attemperatures of up to about 300° F., or alternatively from about 150° F.to about 200° F.). Where desired, curing characteristics (e.g., curingtime, etc.) may be adjusted to fit particular layer application methodsand/or final product specifications by, for example, adjusting relativeamounts of resin components. Still further examples of suitable resinsand coating methods include, but are not limited to, those found inEuropean Patent Application EP 0 771 935 A1; and in U.S. Pat. Nos.4,869,960; 4,664,819; 4,518,039; 3,929,191; 3,659,651; and 5,422,183,each of the foregoing references being incorporated herein by referencein its entirety.

With benefit of this disclosure, those of skill in the art willunderstand that first and one or more second materials may be selectedto meet particular criteria based on the information and examplesdisclosed herein, as well as knowledge in the art. In this regard, oneor more second material coatings or layers may be present, for example,to substantially protect the ground walnut hull first material fromdownhole fluids such as formation, drilling, workover fluids (e.g., saltwater, acid, etc.), and/or to harden or otherwise modify the firstmaterial from closure stress or other mechanical stresses that may beencountered downhole. In this regard, thickness or amount of one or morecoatings may be any amount suitable to provide a particle having analtered in situ Young's modulus and/or to provide at least partialprotection, for the inner first material, from wellbore or formationconditions.

In one embodiment, a coating of one or more second materials may be fromabout 0.1% by weight to about 50%, alternatively from about 1% by weightto about 20% by weight, alternatively from about 10% by weight to about20%, alternatively from about 2% to about 12% by weight of the totalweight of the multi-component particle, although greater and lesseramounts are possible. In this way, a first material such as groundwalnut shell particulates may be coated with, for example, from about 2%to about 12% of a suitable resin (e.a., BORDEN “SIGMASET LV” resin) byweight of total weight of each particle to form relatively lightweightand/or substantially neutrally buoyant proppant particulate. Suchparticles may exhibit increased strength and/or resistance to wellfluids over uncoated ground walnut hulls. In one embodiment, it has beenfound that application of from about 8% to about 12% by weight of totalparticle weight of “SIGMASET LV” resin to ground walnut hull particulatematerial serves to permeate the material so as to substantially fill theaccessible or permeable porosity of the materials such that a relativelyshiny or glazed surface appearance is achieved.

In one exemplary embodiment, about 12/20 mesh ground walnut hulls from“COMPOSITION MATERIALS, INC.” having an in situ Young's modulus of fromabout 1,000,000 psi to about 2,000,000 psi (and described elsewhereherein) may be coated with a second material, such as “SIGMASET LV” or“SIGMASET XL” resin available from BORDEN CHEMICAL (in amounts asdescribed elsewhere herein). Such coated particles may be manufacturedand/or supplied, for example, by BORDEN CHEMICAL. It will be understoodthat a protective resin layer may also function as a hardener to thecore material, however, an additional and separate hardener materiallayer may also be present to impart additional hardness to the corematerial if so desired. In one exemplary embodiment in which such aseparate hardener layer is present, ground walnut shell particulates maybe first coated with from about 2% to about 10% by weight (andalternatively about 2% by weight) of total weight of a separate hardenermaterial (e.g., BORDEN “2AC” hardener) and then coated with from about1% to about 20% by weight (and alternatively about 4% by weight) ofanother resin (e.g., BORDEN “SIGMASET XL” or “SIGMASET LV” resin). Inone exemplary embodiment then, the 12/20 mesh ground walnut shellsdescribed above may be coated with about 2% by weight of total weight ofBORDEN “2AC” hardener and about 4% by weight of total weight of BORDEN“SIGMASET XL.”

It will be understood that the coating amounts given herein areexemplary only, and may be greater or lesser, and that amounts and typesof core, separate hardener material and/or other protective layermaterial/s may be selected with benefit of this disclosure by those ofskill in the art to meet or and withstand anticipated downholeconditions of a given application using methods known in the art, suchas those described herein (e.g., in Examples 1 and 2). For example, inthe embodiment above, ground walnut shell particles having about 2% byweight “SIGMASET XL” may be employed for relatively lower closure stressapplications (such as some sand control applications), and ground walnutshell particles having closer to about 10% by weight “SIGMASET XL” maybe employed for relatively higher closure stress applications (such as aproppant or fracture pack particulate), although it will be understoodthat these are exemplary guidelines only.

In one embodiment, the second material coating may be present, forexample, to substantially protect the ground walnut hull first materialfrom downhole fluids such as formation, drilling, workover fluids (e.g.,salt water, acid, etc.), while at the same time altering the in sitiYoung's modulus of the particles from a starting value of about1,000,000 psi to about 2,000,000 psi, to an overall value of from about2,000,000 to about 3,000,000 psi.

In another exemplary embodiment, ground walnut hulls (or another porousfirst material) may be partially or completely impregnated with a secondmaterial, by for example, vacuum and/or pressure impregnation, sprayingwith hardener, or a combination thereof. For example, material may beimmersed in a second material and then exposed to pressure and/or vacuumto impregnate the material. Such methods are known in the art forimpregnating porous materials, such as impregnating core samples withfluids, etc. Alternatively, application of a second material may resultin at least partial impregnation, for example, it has been found that upto about 10% to about 12% by weight of total particle weight of resin(such as BORDEN “SIGMASET XL”) may be applied and penetrate into theporosity of ground walnut shells. Furthermore, it will be understoodthat a first relatively lightweight and/or substantially buoyantmaterial may be combined with more than one other material, e.g., usingthe methods and configurations described elsewhere herein forembodiments involving first and second materials.

It will be understood with benefit of the disclosure that any othermaterial suitable for coating a substantially hard proppant core andhaving suitable protective, hardening, and/or specific gravity-alteringcharacteristics as defined elsewhere herein may be employed.

Although embodiments of the disclosed method employing layeredmulti-component particles having two components or layers have beendescribed and illustrated above, it will be understood that otherconfigurations of layered multi-component relatively lightweight and/orsubstantially neutrally buoyant particles may be employed. For example,layered particles may include a core with two or more layers ofmaterials surrounding the core. Any combination of two or more materialsmentioned elsewhere herein may be employed in multi-component particleshaving a core surrounded by two or more layers. In this regard,particles having two or more layers of materials may be useful forproviding desirable properties.

Manufacture of the disclosed embodiments of multi-component particlesmay be by any suitable method known in the art. In this regard, one ormore layers of coatings may be applied using any coating method known inthe art to a selected embodiment of core material described elsewhereherein. Coatings may be applied directly, or where required ordesirable, binder materials/compositions known to those of skill in theart may be used to enhance ease of application or to enhance integrityof an applied layer/s to a core or underlying layer of selectedmaterial.

EXAMPLES

The following examples are illustrative and should not be construed aslimiting the scope of the invention or claims thereof.

Example 1: Resin-Coated Ground Walnut Shells

Conductivity tests were performed according to API RP 61 (1^(st)Revision, Oct. 1, 1989) using an API conductivity cell with Ohiosandstone wafer side inserts. Each particulate material sample wasloaded into the cell and closure stress applied to the particulatematerial using a “DAKE” hydraulic press having a “ROSEMOUNT”differential transducer (#3051C) and controlled by a “CAMILE”controller. Also employed in the testing was a “CONSTAMETRIC 3200”constant rate pump which was used to flow deionized water through eachparticulate sample.

The coated ground walnut particulate material employed was ground walnuthulls from “COMPOSITION MATERIALS, INC.” having a size of about 12/20mesh and having an in situ Young's modulus of from about 1,000,000 psito about 2,000,000 psi. The ground walnut particulate material wascoated with a layer of BORDEN “SIGMASET LV” low volatility resin in anamount of about 12% by weight of total particulate weight, and theparticles were manufactured by “BORDEN CHEMICAL”. The coated groundwalnut particulate material was tested alone, with no other particulatematerial blended in. It will be understood with benefit of thisdisclosure that other particles having a similar modulus describedelsewhere herein (e.g., ground or crushed nut shells, ground or crushedseeds, etc.) may also be employed in such applications as the soleproppant component of a fracturing fluid.

Experimental parameters for the coated walnut shell conductivityevaluation are shown in Tables I-III below.

TABLE I Fluid Deionized Water Particulate (grams) 63 Top Core (cm) 0.91Bot Core (cm) 0.968 Initial Total Width (cm) 5.462 Width Pack, initial(cm) 1.134

TABLE II Temperature 150 Particulate Size 12/20 Closure Pressure500-2000 psi Concentration   2 lbs/ft2 Fluid Pressure (psi) 387 Baseline 238 Darcies @ 1000 psi

TABLE III Test Water Data Rate Visc- Conduct- Permea- Closure *Time Tempmls/ osity DP Width ivity bility Stress (Hours) ° C. min cp psi inchesmd-ft darcies psi 0 68.45 7.89 0.41 0.00386 0.433 22,608  626  524 1065.20 16.27 0.43 0.01195 0.427 15,756  442  456 20 65.19 7.73 0.430.00613 0.406 14,585  432 1001 30 65.15 7.80 0.43 0.01445 0.355 6,251211 2029 40 65.21 7.87 0.43 0.01469 0.351 6,203 212 2019 50 65.21 7.820.43 0.01483 0.348 6,106 211 2021 70 65.22 7.79 0.43 0.01516 0.346 5,947206 2021 *Values given represent an average of an hour's data at eachgiven point.

As may be seen from the results of this example, a relativelylightweight particulate that is substantially neutrally buoyant in a 10pound per gallon brine, may advantageously be employed to yield aproppant pack having relatively good conductivity. At 1,000 psi closurestress the pack of relatively lightweight proppant material exhibitedpermeabilities equal to or exceeding any of the conventional proppants(sand, etc.).

Example 2: Ground Walnut Shells Coated with Various Resins

Using a procedure similar to that of Example 1, the same type of 12/20mesh ground walnut hull core material was tested with different types ofresin layers from BORDEN. Testing was carried out for all samples at150° F. and closure stresses ranging from 500 psi to 2000 psi. For twoof samples, testing was also carried out at 200° F. and closure stressof 2200 psi. Resin type and amounts used in each sample are identifiedin Table IV. Results of this testing is given in Tables V and VI, and inFIGS. 2 and 3.

TABLE IV BORDEN Resin Layers on 12/20 Mesh Ground Walnut Shell MaterialLayer Type and Amount (% by Weight of Total Weight Sample Identifier ofParticle)* A Inner layer of 2% by weight BORDEN “2AC” with Outer Layerof 4% by weight BORDEN “SIGMASET LV” B Layer of 6% by weight BORDEN“SIGMASET LV” resin (Coated particles having Borden identification code“66040”) C Layer of 6% by weight BORDEN “SIGMASET LV” resin (Coatedparticles having Borden identification code “66535”) D BORDEN Two CoatResin — Inner layer of 2% by weight separate hardener material and outerlayer of 3% by weight “SIGMASET LV” (Coated particles having Bordenidentification code “2PN3x”) E Layer of 12% by weight BORDEN “SIGMASETLV” *— In Table IV, BORDEN product identification codes 66040 and 66535denote particles coated with “SIGMASET LV” resin having modified curingcharacteristics, i.e., the first digit in the code represents the % byweight of resin applied as a percentage of total particle weight (e.g.6%), the second and third digits in the code represent weight percentageof the first resin component (e.g., 60% and 65% respectively), and thefourth and fifth digits represent weight percentage of the # secondresin component (e.g., 40% and 35% respectively).

TABLE V Closure Permeability, Darcies Stress, Sample Sample SampleSample psi Sample A B C D E 500 453 205 383 429 432 1000 303 146 200 153319 2000 220 46 94 88 206 105 76

TABLE VI Cell Width, Inches Closure Sample Sample Sample Sample Stress,psi Sample A B C D E 500 0.43 0.43 0.41 0.41 0.43 1000 0.41 0.4 0.380.39 0.406 2000 0.36 0.345 0.3 0.35 0.35 2200 0.32 0.299

FIG. 2 shows the permeability of the relatively lightweight particulatecore materials having the various types of resin layers of this exampleat 500, 1000 and 2000 psi closure stresses and 150° F.

FIG. 3 shows pack or conductivity cell width of the relativelylightweight particulate core materials having the various types of resinlayers of this example at 500, 1000 and 2000 psi closure stresses and150° F. Also shown is cell or pack width of the relatively lightweightparticulate materials Samples A and E at 2200 psi closure stress and200° F.

The results of Examples 1 and 2 illustrate just one way that relativelylightweight particulate core materials may be evaluated with varioustypes and/or amounts of resins to fit particular conditions, forexample, anticipated wellbore or formation conditions. With benefit ofthis disclosure, those of skill in the art will understand that usingthis or other methods known in the art suitable for simulatinganticipated downhole conditions, types of relatively lightweightmaterial core materials and coatings (or combinations of two or morecoatings) may be selected or tailored for use in a given desiredapplication.

While the invention may be adaptable to various modifications andalternative forms, specific embodiments have been shown by way ofexample and described herein. However, it should be understood that theinvention is not intended to be limited to the particular formsdisclosed. Rather, the invention is to cover all modifications,equivalents, and alternatives falling within the spirit and scope of theinvention as defined by the appended claims.

What is claimed is:
 1. A method of fracturing a subterranean formation,comprising: injecting a particulate material into said subterraneanformation at a pressure above the fracturing pressure of said formation;wherein at least a portion of said individual particles of saidparticulate material each comprise a first material selected from atleast one of ground or crushed nut shells, ground or crushed seedshells, ground or crushed fruit pits, processed wood, or a mixturethereof; and wherein at least a portion of said individual particles ofsaid particulate material each comprises a core component of said firstmaterial at least partially surrounded by at least one layer componentof second material, said second material comprising a protective orhardening coating.
 2. The method of claim 1, wherein said secondmaterial comprises at least one of phenol formaldehyde resin, melamineformaldehyde resin, urethane resin, or a mixture thereof.
 3. The methodof claim 2, wherein said individual particles of said particulatematerial further comprise a third material hardener applied between saidfirst and second materials.
 4. The method of claim 2, wherein saidindividual particles of said particulate material comprise at least oneof ground or crushed walnut shells, ground or crushed ivory nut shells,ground or crushed peach pits, ground or crushed apricot pits, or amixture thereof.
 5. The method of claim 4, wherein said first materialcomprises ground or crushed walnut shells; and wherein said secondmaterial comprises urethane resin.
 6. The method of claim 5, whereinsaid individual particles of said particulate material further comprisea third material hardener applied between said first and secondmaterials.
 7. The method of claim 6, wherein said particles have aparticle size of from about 4 mesh to about 100 mesh; and wherein saidlayer component of material comprises from about 1% to about 20% byweight of the total weight of each of said individual particles of saidparticles.
 8. The method of claim 7, wherein said individual particlesof said particulate material are injected into said formation assubstantially neutrally buoyant particles in a carrier fluid.
 9. Themethod of claim 8, wherein said carrier fluid is an ungelled aqueousfluid, or an aqueous fluid characterized as having a polymerconcentration of from greater than about 0 pounds of polymer perthousand gallons of base fluid to about 10 pounds of polymer perthousand gallons of base fluid, and as having a viscosity of from about1 to about 10 centipoises.
 10. The method of claim 2, wherein at least aportion of said individual particles of said material comprise a porouscore component that is impregnated with said second material.
 11. Themethod of claim 2, wherein said layer component of material comprisesfrom about 1% to about 20% by weight of the total weight of each of saidindividual particles of said particles.
 12. The method of claim 2,wherein an in situ temperature of said subterranean formation is lessthan or equal to about 150° F. and an in situ closure of stress of saidsubterranean formation is less than or equal to about 1500 psi.
 13. Themethod of claim 1, wherein individual particles of said particulatematerial have a shape with a maximum length-based aspect ratio of equalto or less than about
 5. 14. The method of claim 1, whereinsubstantially all of the individual particles of said particulatematerial introduced into said well comprise a core component of saidfirst material and a layer component of said second material.
 15. Amethod of fracturing a subterranean formation, comprising: introducing aparticulate material suspended in a carrier fluid into said subterraneanformation at a pressure above a fracturing pressure of said subterraneanformation; wherein at least a portion of the individual particles ofsaid particulate material are substantially neutrally buoyant in saidcarrier fluid and comprise: a core component of a first materialselected from at least one of ground or crushed nut shells, ground orcrushed seed shells, ground or crushed fruit pits, processed wood, or amixture thereof, and at least one layer component of second materialsurrounding said core component, said second material comprising aprotective or hardening coating.
 16. The method of claim 15, whereinsaid second material comprises at least one of phenol formaldehyderesin, melamine formaldehyde resin, urethane resin, or a mixturethereof.
 17. The method of claim 16, wherein said individual particlesof said particulate material further comprise a third material hardenerapplied between said first and second materials.
 18. The method of claim16, wherein said carrier fluid is an ungelled aqueous fluid, or anaqueous fluid characterized as having a polymer concentration of fromgreater than about 0 pounds of polymer per thousand gallons of basefluid to about 10 pounds of polymer per thousand gallons of base fluid,and as having a viscosity of from about 1 to about 10 centipoises. 19.The method of claim 18, wherein said particulate material has a specificgravity of from about 1.25 to about 1.35, and wherein said carrier fluidhas a specific gravity of between about 1 and about 1.5.
 20. The methodof claim 19, wherein said wellbore has an angle with respect to thevertical of between about 30 degrees and about 90 degrees.
 21. Themethod of claim 15, wherein said individual particles of saidparticulate material comprise at least one of ground or crushed walnutshells, ground or crushed ivory nut shells, ground or crushed peachpits, ground or crushed apricot pits, or a mixture thereof; and whereinsaid second material comprises at least one of phenol formaldehyderesin, melamine formaldehyde resin, urethane resin, or a mixturethereof.
 22. The method of claim 15, wherein said carrier fluid is anungelled aqueous fluid, or an aqueous fluid characterized as having apolymer concentration of from greater than about 0 pounds of polymer perthousand gallons of base fluid to about 10 pounds of polymer perthousand gallons of base fluid, and as having a viscosity of from about1 to about 10 centipoises.
 23. The method of claim 15, wherein saidfirst material comprises ground or crushed walnut shells; wherein saidsecond material comprises urethane resin; and wherein said individualparticles of said particulate material further comprise a third materialhardener disposed between said first and second materials.
 24. Themethod of claim 15, wherein substantially all of the individualparticles of particulate material contained in said carrier fluid aresubstantially neutrally buoyant in said carrier fluid, and comprise acore component of said first material and a layer component of saidsecond material.